Recovery of hydrocarbons from polyalkene product purge gas

ABSTRACT

Hydrocarbons are recovered from the product purge gas in an alkene polymerization process by absorption of heavier hydrocarbons from the purge gas by an intermediate hydrocarbon stream to yield a vapor rich in inert gas and alkene monomer. Alkene monomer is condensed at low temperatures from the inert gas, flashed and vaporized to provide refrigeration for the condensation step, and recycled to the polymerization process. The intermediate hydrocarbon from the absorption step is recycled to the polymerization process. Optionally a portion of the inert gas is reused for purge gas.

FIELD OF THE INVENTION

The invention pertains to the production of polyalkenes such aspolyethylene and polypropylene, and in particular to the recovery ofhydrocarbons from polyalkene product purge gas.

BACKGROUND OF THE INVENTION

In the synthesis of polyalkenes such as polyethylene and polypropylene,solid particles of polymer product are withdrawn from the reactor withinterparticle gaseous components present in the reactor. The solidpolymer product is purged with an inert gas such as nitrogen before theproduct is pelletized, yielding a nitrogen-rich purge gas containing anumber of hydrocarbon compounds including unreacted ethylene orpropylene, some alkanes such as ethane or propane, comonomers such asbutene-1 and hexene-1, additives such as isobutane or isopentane whichare added to the reactor feed as diluents or moderators, and smallamounts of oligomers such as octene, decene, or heavier olefins. Thehydrocarbon content of this product purge gas can range up to 50 mol% orhigher, most of which consists of the valuable feed components ethyleneor propylene. Recovery of these feed components is economicallydesirable, as is recovery of the comonomers and additives if present.Significant amounts of heavier components such as oligomers, if presentin the purge gas, will be detrimental to adsorption or low temperaturecondensation methods for recovering the valuable lighter components.

Adsorption processes have been proposed to separate hydrocarbons frompurge or vent gas streams produced by processes similar to polyethyleneand polypropylene reactor systems. U.S. Pat. No. 3,266,221 discloses apressure swing adsorption unit which uses molecular sieves to separatehydrocarbons, primarily ethylene and propylene, from byproduct carbondioxide and diluent nitrogen in a direct oxidation process for theproduction of ethylene oxide. The recovered hydrocarbons are thencompressed, purified and recycled to the reaction zone.

U.S. Pat. No. 4,769,047 discloses an improvement to that process wherebythe ethylene oxide vent gas stream is first contacted with activatedcarbon to remove the propylene and higher hydrocarbons which have adeleterious effect when recycled to the ethylene oxide reactor with theethylene. The ethylene is then recovered in a downstream pressure swingadsorption unit, purified by conventional liquid absorption methods toremove residual carbon dioxide, compressed and recycled to the ethyleneoxide reactor system. Since heavy oligomers such as octene and deceneare not formed in the ethylene oxide reactor system, there is no problemof permanent adsorption of these materials in the pressure swingadsorption units proposed in these patents. Potential problems withother heavy materials which might be present in the vent gases are notaddressed in either of these patents.

Pressure swing adsorption can be utilized to recover ethylene andpropylene from the nitrogen purge gas used in polyethylene andpolypropylene production processes. Most of the ethylene and propylenecan be removed from the nitrogen so that a lower quantity ofhydrocarbons is discharged to the atmosphere with the nitrogen. However,the purity of the recovered ethylene and propylene is too low, that is,it contains too much nitrogen, to be recycled to the reactor system andinstead must be burned as fuel or otherwise disposed of.

U.S. Pat. No. 3,336,281 discloses an improvement to a high pressureprocess for the manufacture of ethylene copolymers operating at 7350psia or above in which ethylene off-gas from a high pressure copolymerproduct separator at 2940 psia or above is cooled to a temperature of 0°C. to -65° C. (+32° F. to -85° F.) by expansion to atmospheric pressure.As a result of this expansion cooling step, a large fraction of theundesirable low molecular weight by-products in the off-gas arecondensed and removed and the uncondensed ethylene can be recycled tothe reactor system. The improvement of this invention is directedprimarily to the removal of the undesirable low molecular weightby-products from a recycle ethylene stream; the separation of ethylenefrom light gases such as nitrogen is not addressed. The potentialproblems of freeze-out of byproducts or comonomers in the cold equipmentare not addressed in this patent.

U.S. Pat. No. 4,217,431 discloses an improvement to a high pressureprocess for the manufacture of ethylene copolymers operating at 20,000to 75,000 psia in which off-gas from a low pressure copolymer productseparator at 150 psia or less is compressed and cooled to -10° C. to-30° C. (+14° F. to -22° F.). As a result of this cooling step, a largefraction of the comonomer in the off-gas, such as vinyl acetate, iscondensed and can be recycled to the reactor system. However, most ofthe ethylene in the off-gas, typically 60 to 70%, must be dischargedfrom the system. The improvement of this invention is directed primarilyto the recovery of the comonomer for reuse; the separation of ethylenefrom light gases is not addressed in the patent. The potential problemsof freeze-out of byproducts or comonomers in the cold equipment are notaddressed in this patent.

U.S. Pat. No. 5,233,060 discloses an absorption/stripping process torecover ethylene from argon purge gas in a direct oxidation process forthe production of ethylene oxide. The ethylene is absorbed in a highmolecular weight organic liquid, such as n-dodecane, and subsequentlystripped from the liquid with nitrogen or methane. The stripped ethyleneis then recycled to the reactor system, along with the nitrogen ormethane stripping gas. Recycling of the nitrogen or methane strippinggas is acceptable in this ethylene oxide process since these gases arenormally used as diluent or ballast gases in the reactor. Potentialproblems with accumulation of heavy materials in the scrub liquid(because these materials can only be partially stripped from the scrubliquid) are not addressed.

A paper by A. Fatemizadeh and E. Nolley presented to the AmericanInstitute of Chemical Engineers, Orlando, Fla., Mar. 19-23, 1990entitled "Ethylene Recovery from Polyethylene Plant Vent Streams"describes in generic terms a cold box concept which uses expanderrefrigeration for ethylene recovery from purge gas streams containingless than 20 mole % nitrogen and other light gases. Specific details ofthe cold box process adequate removal of light gases. The processrequires an expansion turbine and is limited to gas streams containingless than 20 mole % nitrogen. A conventional lean oil absorption processis described for recovering ethylene from such a stream in whichethylene-containing oil is regenerated and recirculated to theabsorption tower.

Improved methods for the recovery of ethylene or propylene fromnitrogen-rich purge gas in the production of polyethylene orpolypropylene are economically desirable. The presence of heavierhydrocarbons in the purge gas can adversely affect adsorption orcondensation processes used for light hydrocarbon recovery, and methodsfor removing such heavy hydrocarbons are needed to allow recovery ofvaluable ethylene or propylene. The invention disclosed below anddefined in the claims which follow addresses these problems and offersimproved purge gas treatment and light hydrocarbon recovery.

SUMMARY OF THE INVENTION

The invention is a method for the recovery of hydrocarbons from productpurge gas (23) generated in the synthesis of a polyalkene product from afeed gas (1) comprising alkene monomer and one or more intermediatehydrocarbons in a polymerization reactor (103). The product purge gas(23) is generated by purging the granular polyalkene product with aninert gas (21). The product purge gas (23) comprises inert gas,unreacted alkene monomer, one or more heavy hydrocarbons formed asbyproducts in the polymerization reactor, and one or more intermediatehydrocarbons having volatilities intermediate the alkene monomer and theone or more heavy hydrocarbons. The method comprises contacting theproduct purge gas (23) in a gas-liquid absorption zone (117) with ascrub liquid feed (31) comprising at least one of the one or moreintermediate hydrocarbons, thereby absorbing at least a portion of theone or more heavy hydrocarbons from the product purge gas (23) into thescrub liquid (31). A vapor overhead (35) comprising inert gas and atleast a portion of the alkene monomer is withdrawn from the absorptionzone (117) and an absorber bottoms scrub liquid (33) comprising at leasta portion of the one or more heavy hydrocarbons is also withdrawntherefrom. The alkene monomer is recovered from the vapor overhead (35),and at least a portion of the absorber bottoms scrub liquid (33) isreturned to the polymerization reactor (103).

The alkene monomer is ethylene or propylene, and the one or moreintermediate hydrocarbons typically are selected from the groupconsisting of isobutane, isopentane, butene-1, hexane, and hexene-1. Thescrub liquid (31) typically comprises a hydrocarbon selected from thegroup consisting of isobutane, isopentane, butene-1, hexane, andhexene-1. The one or more heavy hydrocarbons include olefins having acarbon number of 8 or greater. The inert gas (21) preferably isnitrogen.

The recovery of alkene monomer is accomplished in one embodiment bycooling and partially condensing the vapor overhead (35) from thegas-liquid absorption zone by indirect heat exchange with cold processstreams, and separating the resulting partially condensed stream (36)into a first vapor (45) rich in inert gas and a first hydrocarbon-richliquid (37). The first hydrocarbon-rich liquid (37) is flashed to areduced pressure and the resulting flashed stream (38) optionally isseparated into a second vapor (51) rich in inert gas and a secondhydrocarbon-rich liquid (39). The resulting flashed firsthydrocarbon-rich liquid (38) or the second hydrocarbon-rich liquid (39)is warmed and vaporized to provide by indirect heat exchange at least aportion of the refrigeration required to cool and partially condense thevapor overhead (35), thus yielding a hydrocarbon-rich vapor (41). Thehydrocarbon-rich vapor (41) is compressed, cooled, and condensed, andthe resulting stream (42) is separated into a monomer-rich vapor (43)and a condensate (55) which contains at least one of the one or moreintermediate hydrocarbons in the scrub liquid. The monomer-rich vapor(43) can be recycled to the polymerization reactor (103). The condensate(55) can be combined with makeup scrub liquid (57) to provide the scrubliquid feed (31) to the gas-liquid absorption zone (117). The firstvapor (45) rich in inert gas optionally is reduced in pressure, and theresulting reduced-pressure vapor (47) and the second vapor (51) rich ininert gas are warmed by indirect heat exchange with the vapor overhead(35) to yield a first (49) and a second (53) warmed inert gas stream,thereby providing another portion of the refrigeration required to cooland partially condense the vapor overhead (35) in the initial stepearlier described. At least a portion of the first warmed inert gasstream (49) may be reused for purging the polyalkene product. In analternative embodiment, the first vapor (45) rich in inert gas isreduced in pressure and the resulting reduced-pressure vapor (47) iscombined with the second vapor (51) rich in inert gas, and the resultingcombined stream (52) is warmed by indirect heat exchange with the vaporoverhead (35) to yield a warmed inert gas stream (54), thereby providinganother portion of the refrigeration required to cool and partiallycondense the vapor overhead (35) in the initial step earlier described.

In certain modes of operation of the alkene polymerization reactor, theamount of heavier hydrocarbons produced are sufficiently low that theheavier hydrocarbons in the product purge gas (23) are completelysoluble in the condensed monomer and intermediate hydrocarbon liquids(37, 39). The problem of these heavier hydrocarbons freezing out in thecondensation step therefore does not occur, and the earlier-describedabsorption step is not required. In such a situation, the product purgegas (23) is processed directly to condense the monomer and intermediatehydrocarbons by the described heating and cooling steps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowsheet of an embodiment of the present invention.

FIG. 2 is a flowsheet of an alternative embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Product purge gas from polyalkene reactors contains an inert gas such asnitrogen, the unreacted alkene monomers ethylene or propylene, hydrogen,light alkanes such as ethane or propane, and some intermediatehydrocarbons of intermediate volatility such as isobutane, isopentane,butene-1, hexane, and hexene-1. In addition, depending upon reactorsystem operation, the purge gas may contain varying concentrations ofheavier molecular weight oligomers such as octene and decene andpossibly other heavier hydrocarbons. This purge gas can be vented,utilized for fuel value or disposed of by combustion in a flare system.However, it is preferred to recover the valuable alkenes ethylene orpropylene for recycle to the polymerization reactor, and optionally torecover a portion of the inert gas for reuse as high purity purge gas.Such recovery can be complicated by the presence of heavier hydrocarbonswhich can freeze out in low temperature processes for condensing thevaluable alkenes or which can deactivate adsorbents used in adsorptionprocesses for purification of the purge gas. The actual concentrationsand phase behavior of these heavier hydrocarbons are important factorsin the selection and design of purge gas treatment systems. The presentinvention offers several embodiments for the recovery of valuablealkenes from polymerization reactor product gas and the recovery ofinert gas for reuse as high purity purge gas. The purge gas can containlight gases at concentrations between 10 and 80 mole %.

The invention can be understood in detail with reference to theembodiment given in FIG. 1. Reactor system 101 for the synthesis ofpolyethylene or polypropylene is well known in the art and thereforewill be explained only in general terms in the following description.For example, the Unipol process for polyethylene production (licensed byUnion Carbide Corporation) is a widely-used gas-phase catalytic processwhich utilizes a fluid bed reactor at a pressure of about 300 psia. TheUnipol process for the manufacture of polypropylene operates in asimilar manner at a pressure of about 500 psia. In another process suchas the LPE process (licensed by the Phillips Petroleum Company),ethylene and a comonomer are mixed with catalyst in an isobutane slurryand reacted in a loop reactor at about 600 psia.

In a representative polyalkene reactor operation, which in the followingdescription is a gas-phase catalytic polyethylene reactor, feed 1 andrecycle stream 3 are combined, the combined feedstream optionally iscombined further with either or both of the additional recycle streams 5and 7 (defined later), and the resulting reactor feed 9 is introducedinto fluidized bed reactor 103. Catalyst 11 typically is introduced intothe reactor at multiple points as shown. Unreacted gas 13 is combinedwith additional unreacted gas 15 and the combined stream is compressedand cooled to provide recycle stream 3. Granular polyethylene product isdepressurized and withdrawn intermittently as raw product 17 which flowsinto separator 105. Offgas from separator 105 is cooled, compressed, andreturned as stream 15 for recycle to the reactor. Granular polymerproduct and entrained gas flow to discharge tank 107 which is purgedwith nitrogen stream 21, and product purge gas 23 is withdrawntherefrom. Although nitrogen is the preferred purge gas, any inert gascan be utilized as long as it is compatible with the withdrawal andhandling of the polymer product and with downstream processing ofproduct purge gas 23. Granular polymer product 25 also is withdrawntherefrom for pelletization.

Product purge gas 23 typically contains nitrogen, a major fraction ofethylene, some hydrogen and ethane, and some intermediate hydrocarbonsof intermediate volatility such as isobutane, isopentane, butene-1,hexane, and hexene-1. In addition, depending upon reactor systemoperation, the purge effluent may contain heavier molecular weightoligomers such as octene and decene and possibly other heavierhydrocarbons. Typically the hydrocarbon content of product purge gas 23is at least 20 mole% and may range up to 50 mole% or higher, most ofwhich is valuable ethylene. This stream can be vented, flared, or usedas fuel, but preferably the valuable hydrocarbons contained therein arerecovered. This recovery is complicated if the earlier-mentionedoligomers and higher molecular weight hydrocarbons are present therein.To prepare product purge gas 23 for hydrocarbon recovery the stream ispassed through surge vessel 109 to smooth fluctuating flow, compressedto about 200 to 700 psia by compressor 111, and cooled by cooler 113 toyield cooled purge stream 26 at about 80° to 120° F.

In one embodiment of the present invention, purge stream 26 is producedin the synthesis of polyethylene and contains primarily nitrogen,ethylene, and ethane, with smaller concentrations of isopentane (anadditive to the reactor), hydrogen, and light hydrocarbons, and a smallconcentration of byproduct oligomers formed in the reactor. In thisembodiment the oligomers and heavier hydrocarbons, which can freeze atlower temperatures, must be removed before the purge effluent is cooledto these lower temperatures to recover the ethylene. Purge stream 26,now at 200 to 700 psia and 80° to 120° F., passes into separator vessel115 for removal of condensate 27 containing some condensed intermediateand heavier hydrocarbons. Vapor 29 is introduced into scrub column 117and is contacted therein with scrub liquid 31 which absorbs theremaining heavier hydrocarbons and is withdrawn as scrub column bottoms33. Bottoms stream 33 is combined with condensate 27 to provide recyclestream 7 which is returned to reactor 103. Nitrogen-rich overhead vapor35 containing light components and some vaporized scrub liquid passes torecovery system 119 for recovery of valuable ethylene as well asvaporized scrub liquid components.

Scrub liquid 31 is selected to contain one or more intermediatehydrocarbons such as isobutane, isopentane, or comonomers such asbutene-1, hexane, or hexene-1 which otherwise would be introduced to thereactor as an additive with feed 1. These hydrocarbons have molecularweights and volatilities intermediate the alkene monomer ethylene andthe heavier oligomers and hydrocarbons mentioned earlier. In addition,the intermediate hydrocarbons used as scrub liquids should havesufficiently low freezing points that residual amounts in nitrogen-richoverhead vapor 35 will not freeze out at the low temperatures requiredto condense ethylene in downstream processing. Because the scrub liquidis compatible with the operation of reactor 103, scrub column bottomsstream 33 needs no additional process steps to remove absorbed heavierhydrocarbons and the entire stream can be recycled to reactor 103.Alternatively a major portion of bottoms stream 33 can be recycled toreactor 103 and the remaining portion withdrawn as a purge to preventheavy hydrocarbon buildup in the reactor loop.

Recovery system 119 can utilize any appropriate type of separationsystem to remove and recover monomer and residual hydrocarbons in scrubcolumn overhead 35. For example, refrigeration can be used to condensethe monomer and residual hydrocarbons for return to the reactor system.Alternatively, the monomer and residual hydrocarbons can be removed andrecovered by selective adsorption and desorption in a pressure swingadsorption process. In either case, prior removal of oligomers andheavier hydrocarbons usually is necessary to prevent buildup of thesecomponents by freezing or permanent adsorption in the downstreamprocessing step.

In a preferred method of the present invention for recovering ethyleneand residual intermediate hydrocarbons from nitrogen-rich scrub columnoverhead 35, the stream is cooled in one or more stages to a temperatureof -150° to -225° F. and partially condensed in heat exchanger 121against cold process streams described later and passed into separatorvessel 123. The problem of heavier components freezing out in exchanger121 does not occur because essentially all of such components werepreviously removed in scrub column 117. Condensed ethylene andadditional light and intermediate hydrocarbons are withdrawn as stream37 which is flashed to about 15 to 50 psia and introduced into separatorvessel 125. Cold condensed liquid 39 is warmed and vaporized inexchanger 121 to provide a major portion of the refrigeration forcondensation of stream 35, and the recovered vapor 41 is recompressed,cooled, and separated in separator vessel 127. Overhead vapor 43withdrawn therefrom, containing primarily ethylene, is returned toreactor 103 as recycle stream 5 earlier described. Additionalrefrigeration for condensation of stream 35 is provided by Joule-Thomson(j-T) expansion of high pressure nitrogen stream 45 from separatorvessel 123 and passing the cooled expanded stream 47 through exchanger121 to yield warmed nitrogen stream 49. Optionally, a portion of warmednitrogen stream 49 is recycled to provide purge nitrogen 21. Lowpressure flashed nitrogen vapor stream 51 from separator vessel 125 iswarmed in exchanger 121 to provide additional refrigeration, thusyielding warmed nitrogen stream 53 which is vented or flared. Nitrogenstream 49 typically contains 904 or more of the nitrogen in purge gasstream 26 and nitrogen stream 53 contains 104 or less of the nitrogen inpurge gas stream 26.

Pressurized condensate 55, which in this case is essentially isopentanewith minor amounts of other hydrocarbons, is withdrawn from separatorvessel 127 at 80° to 120° F. and 200 to 700 psia, and is combined withadditional scrub column makeup 57 to yield scrub column liquid feed 31previously described. In this embodiment of the invention, isopentane isa desired additive to reactor 103 and is introduced as scrub columnmakeup 57 rather than with reactor feed 1.

In certain modes of operation of reactor system 101, reactor 103 mayproduce amounts of oligomers and heavier hydrocarbons sufficiently lowsuch that the oligomers and heavier hydrocarbons in stream 29 willremain soluble in condensed lighter and intermediate hydrocarbon liquids37 and 39 containing ethylene, ethane, and isopentane which condense inheat exchanger 121. The problem of heavier components freezing outexchanger 121 therefore does not occur. In such a situation, the purgevapor stream from separator vessel 115 is taken as cooled purge stream30 directly to heat exchanger 121 for recovery of ethylene andisopentane. Recovered hydrocarbon stream 41 from exchanger 121 iscompressed and cooled to yield stream 42, which is returned directly toreactor 103 as recycle stream 5. Separator vessel 127 is not required inthis case. Essentially all of the oligomers and heavier hydrocarbons instream 30 are contained in stream 42. In this embodiment of the presentinvention, scrub column 117 and scrub makeup stream 57 are not required,and condensate 27 is returned directly to reactor 103 as stream 7.Optionally, purge gas stream 30 can be cooled initially in heatexchanger 121 to an intermediate temperature where most of the heaviercomponents have condensed, for example +50° to -25° F., and then sent toan intermediate separator vessel. The condensed heavier components areseparated from the uncondensed purge gas in the intermediate separatorvessel and may be recovered directly. Alternatively the condensedheavier components may be rewarmed separately or may be combined withone of the other warming streams in heat exchanger 121. The uncondensedpurge gas from the intermediate separator vessel is than returned toheat exchanger 121 and further cooled to condense the ethylene orpropylene.

An alternative embodiment of the invention is illustrated in FIG. 2.Scrub column overhead 35, or optionally cooled purge stream 30 if scrubcolumn 117 is not required, is cooled and partially condensed in heatexchanger 121 as earlier described, and the resulting stream isseparated in separator 123 to yield condensate 37. Vapor 45 fromseparator 123 is expanded, preferably by J-T expansion across athrottling valve, to yield further cooled vapor 47 liquid 37 is flashedand separated in separator 125 to yield vapor 51 and liquid 39. In thisembodiment, vapor streams 47 and 51 are combined as stream 52 which iswarmed in exchanger 121 to provide additional refrigeration, and theresulting warmed nitrogen 54 is vented or flared. The amount ofhydrocarbons discharged to the atmosphere if nitrogen stream 54 isvented is reduced by about two orders of magnitude compared with ventingof purge gas stream 23. By combining vapor streams 47 and 51, heatexchanger 121 is simpler and operates more efficiently than in theprevious embodiment; this advantage is offset by the possibility thatnitrogen 54 cannot be recycled to purge vessel 107.

In another alternative embodiment, separator vessel 125 is eliminatedand condensate 37, after flashing, is vaporized and warmed in heatexchanger 121. This embodiment also simplifies heat exchanger 121 butresults in a higher level of nitrogen in the recovered ethylene orpropylene. In another embodiment, at least a portion of thenitrogen-rich vapor 45 from separator vessel 123 is warmed in heatexchanger 121 without being reduced in pressure so that nitrogen stream49 can be recovered at a higher pressure for purging or other uses.

To illustrate one embodiment of the invention as described above withreference to FIG. 1, a heat and material balance was carried out fortreating 135 lbmoles per hour of polyethylene product purge gas 23containing 50 mole % nitrogen, 31 mole % ethylene, 13 mole % ethane, and4 mole % isopentane additive, with the remaining 2 mole % comprisinghydrogen, other light hydrocarbons, and 5000 ppmv oligomers. Productpurge gas 23 after pressure damping in surge vessel 109 is compressed to450 psia by compressor 111, cooled to 100° F. in cooler 113, andseparated in separator 115 to yield compressed vapor 29 and condensate27 which is recycled to reactor 103 as part of stream 7. Compressedvapor 29 is contacted with 5.5 lbmoles per hour of isopentane in scrubcolumn 117 which contains five theoretical stages of vapor-liquidcontacting, and essentially all oligomers and other heavy hydrocarbonsare absorbed in the scrub liquid which is withdrawn as scrub columnbottoms 33. Condensate 27 and scrub column bottoms 33 are combined andrecycled as stream 7 to reactor 103. The total concentration ofoligomers and other heavy hydrocarbons in scrub column overhead 35 isless than 50 ppmv, which is sufficiently low that these components willnot freeze out in downstream low-temperature process equipment.

Scrub column overhead 35, which is saturated with about 6 mole %isopentane, passes to recovery system 119 which in this example utilizeslow temperature heat exchanger 121 to cool and partially condenseoverhead stream 35 as earlier described. Recovered vapor 41 from heatexchanger 121 contains 62 mole % ethylene, 25 mole % ethane, 12 mole %isopentane, and less than 1 mole % nitrogen and other light gases. Vapor41 is compressed to 450 psia, cooled, and recovered as recycle stream 43which contains all the ethylene and ethane and about half of theisopentane from stream 41. Stream 43 is recycled to reactor 103 asstream 5. Condensed isopentane stream 55 is combined with 1.6 lbmolesper hour of isopentane in makeup stream 57 and the combined stream 31 isreturned to scrub column 117. Nitrogen stream 49, which contains about 1mole % ethane and ethylene, optionally is recycled to provide purgenitrogen 21; otherwise the stream is vented or combusted in the plantflare system. Nitrogen stream 53, which contains higher concentrationsof ethane and ethylene than stream 49, is combusted in the plant flaresystem.

In the alternative mode of operation of reactor system 101 discussedearlier, reactor 103 may produce very low amounts of oligomers and heavyhydrocarbons such that these oligomers and heavier hydrocarbons instream 26 will be soluble in condensed liquids containing ethylene andintermediate hydrocarbons. This embodiment of the present invention isillustrated by the following example. Referring to FIG. 1, 100 lbmolesof product purge gas 23 withdrawn from purge vessel 107 contains 57 mole% nitrogen and 41 mole % ethylene with the remainder consisting of lighthydrocarbons and hydrogen. The purge gas also contains hexene-1comonomer and oligomers such as C₈ and heavier olefinic byproducts ofthe polymerization reaction in reactor 103. Product purge gas 23 iscompressed to 400 psia and cooled to 100° F. in cooler 113. Compressedand cooled purge gas 26 is taken directly as stream 30 to heat exchanger121 and cooled against cold process streams to -200° F. The resultingcooled and partly condensed stream is separated in separator vessel 123into ethylene-rich liquid 37 and nitrogen-rich vapor 45 which containsless than 1.5 mole % ethylene and other hydrocarbons. Liquid 37 contains98% of the ethylene in purge gas 30, and also contains dissolved heavyhydrocarbons such as hexene-1 and oligomers at concentrations belowtheir respective solubility limits. Nitrogen-rich vapor 45 is reduced inpressure to 115 psia, warmed in exchanger 121 to 83° F., and vented tothe atmosphere or recycled as stream 21. Ethylene-rich liquid 37 isflashed to 20 psia and passed into separator vessel 125. Dissolvednitrogen is released therein as vapor 51 from ethylene-rich liquid 39,and vapor 51 containing about 8 mole % ethylene is warmed in exchanger121 and combusted in the plant flare. Ethylene-rich liquid 39, whichcontains 98 mole % ethylene and less than 1 mole % nitrogen, isvaporized and warmed to 83° F. in exchanger 121 to provide most of therefrigeration for condensation of the purge gas stream 30. Vaporizedethylene stream 41 is compressed to 400 psia and recycled to reactor103. In this example, 97% of the ethylene in product purge gas 23 isrecovered and recycled to reactor 103.

The description above and examples describe the recovery of ethylene inpolyethylene production, although as earlier stated the method of thepresent invention alternatively can be applied to the recovery ofpropylene in polypropylene production. While the intermediatehydrocarbons, comonomers, and heavy hydrocarbon byproducts may besomewhat different in polypropylene production than in polyethyleneproduction, the process steps in recovering the monomers ethylene andpropylene will be the same. When low temperature heat exchanger 121 isused to recover propylene, the temperature of separator vessel 123typically will be about -50° to -150° F.

Thus the embodiments of the present invention provide improved methodsfor recovering the light hydrocarbons ethylene or propylene frompolyalkene product purge gas for recycle to the polymerization reactor.In one embodiment of the invention, oligomers and heavier hydrocarbonsare removed by absorption in an intermediate hydrocarbon which iscompatible with the polyalkene reactor operation. This eliminates theproblems caused by freezout of the oligomers and heavier hydrocarbonswhen low temperature condensation is used for the recovery of light andintermediate hydrocarbons from the purge gas. This embodiment alsoeliminates the problem of adsorbent deactivation or permanent adsorptionwhen adsorption-based methods are used to recover light and intermediatehydrocarbons from the purge gas. Because the intermediate hydrocarbonused for absorption is compatible with the polyalkene reactor operation,the absorption step can be operated on a once-through basis and aseparate capital- and energy-intensive step for recovery of ethylene orpropylene from the absorbing liquid is not required. The rich absorberbottoms liquid is recycled directly to the polymerization reactor. Adistinguishing feature and advantage of the alkene condensation andrecovery steps described earlier is that a large portion of therefrigeration for condensing the light and intermediate hydrocarbons isprovided through autorefrigeration by flashing and vaporizing thecondensate itself before recycle as vapor to the polyalkene reactor.Additional refrigeration can be provided by J-T expansion and warming ofthe nitrogen-rich vapor stream(s) before recycle or flaring, so thatexpensive external refrigeration such as that provided by an expander isnot required. Alternatively, inexpensive auxiliary refrigeration couldbe provided so that olefin-rich liquid stream 39 can be vaporized andrecovered at higher pressures, for example above 50 psia. This auxiliaryrefrigeration could be obtained from any convenient source such as thevaporization of liquid ethylene or propylene which is used as afeedstock in the plant or the vaporization of liquid nitrogen used asutility or purge gas in the plant. Ethylene or propylene refrigerationmight be obtained from an associated ethylene plant.

The essential characteristics of the present invention are describedcompletely in the foregoing disclosure. One skilled in the art canunderstand the invention and make various modifications thereto withoutdeparting from the basic spirit thereof, and without departing from thescope of the claims which follow.

We claim:
 1. A method for the recovery of hydrocarbons from productpurge gas (23) generated in the synthesis of a polyalkene product from afeed gas (1) comprising alkene monomer and one or more intermediatehydrocarbons in a polymerization reactor (103), wherein said productpurge gas (23) is generated by purging said polyalkene product with aninert gas (21), and wherein said product purge gas (23) comprises saidinert gas, unreacted alkene monomer, one or more heavy hydrocarbonsformed as byproducts in said polymerization reactor, and one or moreintermediate hydrocarbons having volatilities intermediate said alkenemonomer and said one or more heavy hydrocarbons, said method comprisingthe steps of:(a) contacting said product purge gas (23) in a gas-liquidabsorption zone (117) with a scrub liquid feed (31) comprising at leastone of said one or more intermediate hydrocarbons, thereby absorbing atleast a portion of said one or more heavy hydrocarbons in said scrubliquid (31); (b) withdrawing from said absorption zone (117) a vaporoverhead (35) comprising said inert gas and at least a portion of saidalkene monomer and an absorber bottoms scrub liquid (33) comprising atleast a portion of said one or more heavy hydrocarbons; (c) recoveringsaid alkene monomer from said vapor overhead (35); and (d) recycling atleast a portion of said absorber bottoms scrub liquid (33) to saidpolymerization reactor (103).
 2. The method of claim 1 wherein saidalkene monomer is selected from the group consisting of ethylene andpropylene.
 3. The method of claim 1 wherein said one or moreintermediate hydrocarbons are selected from the group consisting ofisobutane, isopentane, butene-1, hexane, and hexene-1.
 4. The method ofclaim 3 wherein said scrub liquid (31) comprises a hydrocarbon selectedfrom the group consisting of isobutane, isopentane, butene-1, hexane,and hexene-1.
 5. The method of claim 1 wherein said one or more heavyhydrocarbons include one or more olefins having a carbon number of 8 orgreater.
 6. The method of claim 1 wherein said inert gas is nitrogen. 7.The method of claim 1 wherein the recovery of said alkene monomer fromsaid vapor overhead (35) is accomplished by selective adsorption anddesorption of said monomer in a pressure swing adsorption process. 8.The method of claim 1 which further comprises compressing and coolingsaid product purge gas (23) and removing any condensate (27) therefromprior to treatment in said gas-liquid absorption zone (117).
 9. Themethod of claim 8 wherein the recovery of said alkene monomer from saidvapor overhead (35) is accomplished by the steps of:(C1) cooling andpartially condensing said vapor overhead (35) by indirect heat exchangewith cold process streams, and separating the resulting partiallycondensed stream (36) into a first vapor (45) rich in said inert gas anda first hydrocarbon-rich liquid (37); (C2) flashing said firsthydrocarbon-rich liquid (37) to a reduced pressure and separating theresulting flashed stream (38) into a second vapor (51) rich in saidinert gas and a second hydrocarbon-rich liquid (39); (C3) warming andvaporizing said second hydrocarbon-rich liquid (39) to provide byindirect heat exchange at least a portion of the refrigeration requiredto cool and partially condense said vapor overhead (35) in step (C1)yielding a hydrocarbon-rich vapor (41); and (C4) compressing, cooling,and partially condensing said hydrocarbon-rich vapor (41) of step (C3) ,separating the resulting stream into a monomer-rich vapor (43) and acondensate (55) which contains at least one of said one or moreintermediate hydrocarbons in said scrub liquid (31).
 10. The method ofclaim 9 which further comprises recycling said monomer-rich vapor (43)to said polymerization reactor (103).
 11. The method of claim 10 whichfurther comprises combining said condensate (55) of step (C4) withmakeup scrub liquid (57) to provide said scrub liquid feed (31) to saidgas-liquid absorption zone (117).
 12. The method of claim 9 whichfurther comprises reducing the pressure of said first vapor (45) rich insaid inert gas, and warming the resulting reduced-pressure vapor (47)and said second vapor (51) rich in said inert gas by indirect heatexchange with said vapor overhead to yield a first (49) and a second(53) warmed inert gas stream, thereby providing another portion of therefrigeration required to cool and partially condense said vaporoverhead (35) in step (C1).
 13. The method of claim 12 which furthercomprises utilizing at least a portion of said first warmed inert gasstream (49) as said inert gas for purging said polyalkene product. 14.The method of claim 9 which further comprises reducing the pressure ofsaid first vapor (45) rich in said inert gas, combining the resultingreduced-pressure vapor (47) and said second vapor (51) rich in saidinert gas, and warming the resulting combined stream (52) by indirectheat exchange with said vapor overhead (35) to yield a warmed inert gasstream (54), thereby providing another portion of the refrigerationrequired to cool and partially condense said vapor overhead (35) in step(C1).
 15. A method for the recovery of hydrocarbons from product purgegas (23) generated in the synthesis of a polyalkene product from a feedgas (1) comprising alkene monomer and one or more intermediatehydrocarbons in a polymerization reactor (103), wherein said productpurge gas (23) is generated by purging said polyalkene product with aninert gas (21), and wherein said product purge gas (23) comprises saidinert gas, unreacted alkene monomer, one or more heavy hydrocarbonsformed as byproducts in said polymerization reactor, and one or moreintermediate hydrocarbons having volatilities intermediate said alkenemonomer and said one or more heavy hydrocarbons, said method comprisingthe steps of:(a) compressing, cooling, and partially condensing saidproduct purge gas (23), and separating the resulting partially condensedstream (26) into a first vapor (30) rich in said inert gas which alsocontains portions of said unreacted alkene monomer, one or more heavyhydrocarbons formed as byproducts in said polymerization reactor, andone or more intermediate hydrocarbons having volatilities intermediatesaid alkene monomer and said one or more heavy hydrocarbons and a firsthydrocarbon-rich liquid (27); (b) further cooling and partiallycondensing said first vapor (30) by indirect heat exchange with coldprocess streams, and separating the resulting partially condensed stream(36) into a second vapor (45) rich in said inert gas and a secondhydrocarbon-rich liquid (37) containing said monomer; (c) flashing saidsecond hydrocarbon-rich liquid (37) to a reduced pressure and separatingthe resulting flashed stream (38) into a third vapor (51) rich in saidinert gas and a third hydrocarbon-rich liquid (39) enriched in saidmonomer; and (d) warming and vaporizing said third hydrocarbon-richliquid (39) to provide by indirect heat exchange at least a portion ofthe refrigeration required to cool and partially condense said firstvapor (30) in step (b), thereby yielding a monomer-rich vapor(41);wherein at least a portion of said one or more heavy hydrocarbonsin said first vapor (30) condense in step (b) and remain soluble in saidsecond (37) and third (39) hydrocarbon-rich liquids.
 16. The method ofclaim 15 which further comprises recycling said monomer-rich vapor (41)to said polymerization reactor (103).
 17. The method of claim 15 whichfurther comprises reducing the pressure of said second vapor (45) richin said inert gas, and warming the resulting reduced-pressure vapor (47)and said third vapor (51) rich in said inert gas by indirect heatexchange with said first vapor (30) to yield a first (49) and a second(53) warmed inert gas stream, thereby providing another portion of therefrigeration required to cool and partially condense said first vapor(30) in step (b).
 18. The method of claim 17 which further comprisesutilizing at least a portion of said first warmed inert gas stream (49)as said inert gas (21) for purging said polyalkene product.
 19. Themethod of claim 15 which further comprises reducing the pressure of saidsecond vapor (45) rich in said inert gas, combining the resultingreduced-pressure vapor (47) and said third vapor (51) rich in said inertgas, and warming the resulting combined stream (52) by indirect heatexchange with said first vapor (30) to yield a warmed inert gas stream(54), thereby providing another portion of the refrigeration required tocool and partially condense said first vapor (30) in step (b).
 20. Themethod of claim 15 wherein said alkene monomer is selected from thegroup consisting of ethylene and propylene.
 21. The method of claim 15wherein said one or more intermediate hydrocarbons are selected from thegroup consisting of isobutane, isopentane, butene-1, hexane, andhexene-1.
 22. The method of claim 15 wherein said one or more heavyhydrocarbons include one or more olefins having a carbon number of 8 orgreater.
 23. The method of claim 15 wherein said inert gas (21) isnitrogen.